An optical image of an object to be photographed which is projected by an objective onto the image plane exhibits the maximum luminous difference or contrast in the image area when the image is precisely focused. Particularly with respect to the edge portion of the brightness distribution of the image, it is observed that the brightness variation curve exhibits the steepest slope when the image is precisely focused. Such phenomenon can be explained by the fact that the light intensity (power spectrum) of the object image with respect to each spatial frequency takes the maximum value when the image is precisely focused. There have already been proposed various automatic focus detecting devices based on such phenomenon. These well known devices can be generally classified into the following two types depending on the method of detection being used with the device. In the well known device of the first type, a plurality of microphotoelectric elements are arranged in the image plane and an output difference between a pair of adjacent ones of these microphotoelectric elements is detected as a contrast signal while, in the well known device of the second type, the object to be photographed is mechanically or electrically scanned to obtain a photoelectrically converted waveform having a series of time-spaced discontinuous peaks. An output is obtained by differentiation of this waveform, which output corresponds to a slope of the image brightness distribution. The output is used as a contrast signal. As an example of the latter, a compact device utilizing a self-scanning photoelectric element as the scanning means has recently been proposed and provided. This self-scanning photoelectric unit comprises a plurality of microphotoelectric elements and a scanner circuit. The unit accumulates a quantity of light to which the series of said microphotoelectric elements are exposed within a single cycle of scanning. The scanner circuit generates a waveform having a series of discontinuous peaks or pulses where amplitudes correspond to the level of light falling on the respective microphotoelectric elements. In this device a series of pulses from the self-scanning photoelectric element is converted to the corresponding analog waveform by using a sample and hold circuit and a filter. The analog signal is then subjected to action of the differentiator to extract the differentiated value from said waveform. This is, in turn, converted by the absolute value circuit to the corresponding absolute value waveform and thereafter the peak value of this output waveform for every cycle of scanning is detected and held. This peak value output is used as the contrast signal with which the focus is indicated or the objective is driven. The self-scanning photoelectric element is advantageous in that an adequate output is obtained even with respect to a low level luminance, since the element of this nature can accumulate a quantity of light within a single cycle of scanning.
Devices of the first type are more convenient in that no movable part is necessary, but it is difficult for this type to detect a low level of light. The well known device of the second type, in which the self-scanning photoelectric element is used and the brightness distribution of the image is differentiated, in spite of the advantage such that even a weak light can be detected under the accumulating action of said element, has the disadvantage that it is difficult for this type to hold the output and the circuit arrangement is necessarily complicated.